1. Field of the Invention
The invention relates to coatings, and in particular to the production of underlayers useful in the suppression of iridescence and in the protection of overlying layers sensitive to alkali metal ions from migration of such ions from an underlying glass surface.
2. Description of the Prior Art
G.B. patent no. 2 031 756B is concerned with thin transparent infra-red reflecting semiconductor coatings which are useful in improving the insulation properties of windows and which, being electrically conductive, can serve as resistance heaters, for example to remove ice or condensation from windows. According to GB 2 031 756B, the use of such coatings has been limited by the fact that they exhibit iridescence colours, especially in reflected light; these iridescence effects are widely regarded as aesthetically unsatisfactory, and the problem is made worse by the variation in iridescence colour which occurs with small variations in coating thickness. GB 2 031 756B proposes to overcome the problem of iridescence by depositing an appropriate iridescence reducing underlayer under the semiconductor coating and recommends, as a preferred form of underlayer, a layer having a refractive index in the range from 1.7 to 1.8 and a thickness in the range 64 nm and to 80 nm. According to GB 2 031 756B, the underlayers may be produced by co-depositing a mixture of components calculated to provide the required refractive index, for example a mixture of 84.+-.3% silicon nitride and the balance silica, referred to as silicon oxynitride. Such silicon oxynitride films may be formed by chemical vapour deposition from a source of silicon (e.g. SiH.sub.4, (CH.sub.3).sub.2 SiH.sub.2, (C.sub.2 H.sub.5) SiH.sub.2, (CH.sub.3).sub.4 Si, SiCl.sub.4, SiBr.sub.4), a source of oxygen (e.g. O.sub.2, H.sub.2 O, N.sub.2 O) and a source of nitrogen (e.g. N.sub.2 H.sub.4, NH.sub.3, HN.sub.3, CH.sub.3 NHNH.sub.2, (CH.sub.3) NNH.sub.2) or a source of both oxygen and nitrogen (NO, NH.sub.2 OH, N.sub.2 H.sub.4 H.sub.2 O) on hot glass at a temperature of 500.degree. C. to 600.degree. C.
However, although there is a requirement for a suitable iridescence reducing underlayer, the underlayers proposed in GB patent no. 2 031 756B have not been used commercially to any significant extent. This may be because of difficulties, in particular the long deposition time required, in producing underlayers of sufficient quality and thickness by the known methods.
GB patent specification no. 2 163 146A is concerned with the production of barrier coatings on a glass surface to prevent migration of alkali metal ions from the glass into an overlying layer sensitive to alkali metal ions, for example indium tin oxide. It describes the production of transparent barrier coatings, having good light transmission and excellent barrier properties by pyrolysis of silane on a hot glass surface above 600.degree. C. in the presence of a gaseous electron donating compound; the presence of the electron donating compound is found to result in incorporation of oxygen from glass into the coating so forming a transparent barrier coating up to 50 nm thick on the glass surface.
The electron donating compounds which may be used in the process of GB patent specification no. 2 163 146A are compounds which contain, either in bonds or as lone pair electrons, electrons which can be donated into the electronic structure of suitable acceptor molecules. The use of the electron donating compound is found to result in the incorporation of oxygen from the glass with silicon from the silane to form the transparent barrier coating on the glass. Although the mechanism is not understood, it is believed to involve adsorption of the electron donating compound on the glass surface. It is preferred to use an electron donating compound which is oxygen free, for example ethylene, or which, although it contains some oxygen, is generally regarded as reducing, for example carbon monoxide and alcohols.
Because the transparent barrier coatings may be prepared in the absence of free oxygen and compounds generally regarded as oxidising agents, the barrier coating may be applied to a ribbon of float glass as it is advanced over the molten metal bath on which it is formed without undue risk of oxidising the molten metal.
Unfortunately, while the use of oxygen free electron donating compound alleviates the risk of the silane being oxidised before it reaches the glass surface and of the reactant gas oxidising a molten metal bath on which the glass ribbon is supported, there is insufficient oxygen availability from the glass for the formation of the thicker iridescence reducing underlayers recommended in GB patent 2 031 756B. While thicker layers may be produced by using oxygen-containing electron donating compounds, for example carbon dioxide, it is found that the use of a combination of silane and carbon dioxide results in either thin coatings of poor durability or, when attempts are made to thicken the coatings, a white hazy deposit.
Moreover, when attempts are made to produce barrier coatings of very high transparency (e.g. a light transmission not less than 2% below that of the base glass) using a combination of silane and ethylene in accordance with GB 2 163 146A, the barrier properties of the coatings were found to be insufficiently consistent for some applications.
There is a need for a process, suitable for commercial operation on a float glass production line, for producing an iridescence reducing underlayer as recommended in GB patent 2 031 756B. There is also a need for a process, suitable for commercial operation on a float glass production line, for producing coatings which are effective as barriers to the migration of alkali metal ions from the glass and have a very high degree of transparency.